It has been known for a long time that the technique of calendering can be used to process PVC mixtures in particular, both plasticized and unplasticized, to give technically high-grade webs of film. A particular difficulty with this technique in the case of polyolefins is to overcome the tendency of the hot polymer melt to stick to the metal surfaces of the calender rolls. With this aim in mind, numerous formulations have been developed in which these sticking characteristics are controlled by adding suitable lubricants and release agents. DE-A-3628322, for example, proposes mixtures of various polyolefins, especially low-pressure polyethylene, PVA (polyvinyl acetate), EVA (ethylene-vinyl acetate) and/or EAA (ethylacrylic acid), in order to increase the processing stability. To solve the processing problem EP-A-0607783 describes mixtures of HDPE having a very low MFR (melt flow index) and linear LDPE (low density) together with a mineral filler.
The tendency of the polymer melt to stick to the hot metal surfaces of the processing machines can also be reduced by utilizing the lubricant effect of surface-active substances, as are employed, for example, as antistatic agents for PVC mixtures. As described in DE-A-2823507, metal sulfonates can be used as release agents for reducing the sticking tendency.
Plasticated polymer compositions which can be processed to webs of film on calendering units are generally required to have a broad softening range and a sufficiently high viscosity at the prevailing processing temperatures and under the prevailing shear conditions.
The high melt viscosities required are possessed by PVC mixtures. High polymers such as polypropylene and polyethylene, however, possess low melt viscosities. Whereas the melt consistency of a PVC molding compound remains relatively unchanged up to the point where the material sticks fast to the rolls and beyond that to the point where the ultimate stability is reached, the viscosity of a polypropylene molding compound undergoes a very sharp decrease until the tack-free time is reached, and especially if the experiment is pursued further, and so considerably hinders processing on calendering units.
For technological and economic reasons (for example, the use of a material of relatively low density or comprising an apolar plastic) there has long been a desire to produce such calendered films on the basis not only of PVC but also of polyolefins, such as homopolymers and copolymers of ethylene or of propylene. Hitherto this was difficult because commercial polyolefin compositions, on calendering, stick fast to the metal rolls after just a very short running time and become very liquid, resulting in films having entirely inadequate physical properties or even in the dripping of the polymer compound from the calender rolls.
Nor did application of the technique common in the calendering of PVC, namely the addition of lubricants whose polarity is adapted to that of the plastic, lead to the desired result for polyolefins. In order to achieve improved processing stability, DE-A-4028407 describes the use of stabilizers; mixtures of phosphites or thio costabilizers and phenols permit somewhat easier processing of the polyolefin molding compound.